1. Field of the Invention
This invention relates to a printing method for printing data as optically readable code on a printing medium by printing plates and also to a printed matter bearing such a printed code.
2. Description of the Related Art
Techniques for printing various data including those of sounds, images and other forms of information as optically readable code on a printing medium such as paper are known. U.S. Pat. No. 5,896,403 discloses such a technique.
FIG. 1 of the accompanying drawing schematically illustrates a dot code 101 printed on a printing medium, which is a sheet of paper. Referring to FIG. 1, the dot code 101 is formed by printing optically readable dots 103 that represent many “1s” or “0s” of binary data, the absence or presence of a dot correspondingly representing a “0” or “1”. The original binary data is restored and, for instance, the original sound is reproduced by optically reading the dot code by a dedicated reading device. For such a dot code to operate properly, each dot of the dot code needs to have a predetermined size and be printed at the proper position. Since each dot is as small as about 30 to 120 μm, a highly sophisticated printing technology is required for printing such dots.
A feasible printing method for printing such dot codes is offset printing. Offset printing is popularly being used because it can print at low cost. However, offset printing is accompanied by a problem referred to as “offset doubling”, which, if occurs, is fatal to dot codes.
This problem of offset doubling will be described in detail by referring to FIGS. 2 and 3 of the accompanying drawing.
FIG. 2 schematically illustrates a printed dot code that is an offset doubling. As an offset doubling is produced in a printing operation, a shadowy dot 107 appears for each of the dots 105 that are proper dots at a position separated by distance d from the proper dot. The shadowy dots 107 are normally formed behind the respective dots as viewed in the printing direction with a density lower than that of the proper dots.
FIG. 3 schematically illustrates how an offset doubling is produced. A color printing machine normally comprises a plurality of printing units, of which only two (a preceding printing unit 200 and a succeeding printing unit 210) are shown in FIG. 3 for the purpose of simplification. The preceding printing unit 200 comprises a blanket cylinder 201, an impression cylinder 202 and a plate cylinder 203. Similarly, the succeeding printing unit 210 comprises a blanket cylinder 211, an impression cylinder 212 and a plate cylinder 213. Dots 105 are printed as a sheet of paper 300 is forced to pass between the blanket cylinder 201 and the impression cylinder 202 of the preceding printing unit 200. Then, the sheet of paper 300 where the dots 105 are printed by the preceding printing unit 200 is then forced to pass through the blanket cylinder 211 and the impression cylinder 212 of the succeeding printing unit 210 and a readable image other than the dots 105 is printed on the sheet of paper 300 with ink of a color other than the color of the dots 105.
The dots 105 printed by the preceding printing unit 200 are proper dots. On the other hand, the dots 107 of the offset doubling are printed as part of the ink used for printing the dots 105 by the preceding printing unit 200 is reversely trapped by the blanket cylinder 211 of the succeeding printing unit 210 as so many dots 220 on the blanket cylinder 211 and then transferred onto the sheet of paper 300 immediately following the former sheet of paper 300 at positions displaced from the positions of the respective corresponding proper dots 105 by a distance of d on the sheet of paper 300 immediately following the former sheet of paper 300.
In the case of a color printing machine comprising a plurality of printing units, such a reverse trap occurs partly because the sheet of paper 300 is moved from one printing unit to the next before the ink transferred onto the sheet of paper 300 by the former printing unit dries. It will be appreciated that the problem of offset doubling apparently does not occur if the reversely trapped ink of the dots is not displaced. However, if the printing units are serviced and regulated properly, offset doublings can occur because sheets of paper can expand or shrink and the rate at which they are moved can vary, if only slightly.
In order to eliminate offset doublings, efforts are being made to prevent ink from being reversely trapped by the succeeding printing units by checking the printing units for backlash, regulating the tension, the blanket and the rate of feeding ink and water and so on. However, it is hardly possible to completely eliminate offset doublings.
FIG. 4 of the accompanying drawing schematically illustrates the density distribution of the offset doubling of FIG. 2 as viewed in cross section taken along line A-A′ in FIG. 2.
Referring to FIG. 4, the properly printed dots 105 show the predetermined density. On the other hand, the shadowy dots 107 of the offset doubling show a low density because the amount of ink used for producing each of the shadowy dots 107 is smaller than the amount of ink used for producing the corresponding proper dot 105. However, as the offset doubling occurs, each of the proper dots comes to appear as an oblong dot, which the reading device cannot read correctly. As a result the original data recorded in the form of dot code cannot be reproduced.
Therefore, elimination of offset doublings is a priority problem for printing dot codes by offset printing.
Jpn. Pat. Appln. KOKAI Publication No. 11-263062 proposes a printing method for removing the problem of offset doublings. Now, the proposed printing method will be discussed by referring to FIGS. 5 through 7 of the accompanying drawings.
Referring firstly to FIG. 5, assume that the image to be produced by printing includes (1) continuous gradation (black (hereinafter referred to as “K”), cyan (hereinafter referred to as “C”), magenta (hereinafter referred to as “M”) and yellow (hereinafter referred to as “Y”)), (2) characters/lines (K) and (3) data codes (K). At the time of color separation for preparing plates, two groups of plates including the first group of a first black K1 plate for (1) gradation and (2) characters/lines, a C plate, an M plate and a Y plate and the second group of only a second black K2 plate for (3) data codes are formed. In other words, two separate plates including the K1 plate for gradation and characters/lines and the K2 plate for data codes are used for black (K) printing.
Then, as shown in FIG. 6, the printing operation is conducted by sequentially using the K1 plates, the C plate, the M plate, the Y plate and the K2 plate in the mentioned order. In other words, the cylinder (printing unit) of the K2 plate is used to complete the printing operation in order to eliminate any offset doublings as no printing unit comes after the printing unit of the K2 plate.
FIG. 7 illustrates a method obtained by modifying the above described method. Referring to FIG. 7, the printing operation using the cylinder of the K2 plate for data codes is conducted first and then the K1 plates, the C plate, the M plate, the Y plate are used sequentially after drying the ink used with the K2 plate to realize a similar effect.
Meanwhile, information recording media typically bearing dot codes 101 have been required to be visually appealing in recent years. For instance, bar codes printed in black on a white background have become increasingly unpopular because they are not visually appealing, although they are widely being used currently. The white background may typically be replaced by a yellow background.
However, the printing method disclosed in the above cited Jpn. Pat. Appln. KOKAI Publication No. 11-263062 has disadvantages for multi-color printing when it is used to make printed matters appealing.
For example, assume that a yellow background is used for a black dot code in a printing operation that is conducted by following the sequence shown in FIG. 6. Then, Y ink has to be used in Step SS24 to produce a yellow background that is much larger than the area to be used for printing the dot code in the immediately succeeding step of SS25 using the K2 plate. However, with the listed sequence, there arose a problem that the dots of the dot code were not produced properly nor read correctly by the reading device. As a result of a painstaking investigation, it was found that the problem occurred because the black ink used for the K2 plate was not easily transferred onto the sheet of paper because the Y ink used immediately before for printing the background over the large area had not dried sufficiently. Thus, with the above described printing method, only the inherent color of the sheet of paper can be used for the background of the dot code. Therefore, this method cannot meet the increasing demand for visually appealing dot codes.
On the other hand, while it is possible to print yellow on the dot code with the method of FIG. 7 shown there in the sequence of the printing operation, the dot code needs to be printed and dried in advance. In other words, a sheet of paper needs to be forced pass through a printing machine twice. Therefore, this method is costly and hence not practical.